Abstract: Numerical investigation of buoyancy effect in forced convective heat transfer to supercritical carbon dioxide flowing in a vertical tube was carried out. When the mass flux is low and the wall heat flux is high, the buoyancy effect is obvious, which might redistribute the radial and axial velocity, even M shaped distribution occurred in the radial direction. When the zero-velocity gradient zoon corresponding to the M shaped velocity distribution arises in the edge of viscous layer, the production and convection of eddy will be weakened, resulting in the heat transfer deteriorated. According to the extended calculation based on experimental data, reducing the wall heat flux, adding the mass flux or raising the inlet temperature can relieve the deterioration of heat transfer caused by buoyancy effect in different level.